The parenchymal cells of the liver, generally called hepatocytes, are the major cell types responsible for drug metabolism and are the cells damaged by hepatotoxicants. These cells therefore represent a desirable experimental system for the evaluation of drug metabolism and hepatotoxicity. There are numerous scientific publications on such application of hepatocytes in drug development.
A major problem with drug development is that a large number of drug candidates would fail in the clinic due to nonefficacy or adverse effects, even though drug candidates chosen for clinical trials are selected after extensive research in preclinical studies using laboratory animals and are found to have acceptable efficacy and safety. One of the major reasons is species differences, namely, that human drug effects may not always be the same as that found in laboratory animals.
One of the major species differences is drug metabolism. It is now known that human and nonhuman animals differ in multiple drug metabolizing enzyme pathways, with the most important being differences in P450 monooxygenases. The differences in enzyme pathways cause differences in metabolic fate of a xenobiotic. A drug can be toxic to humans and not to laboratory animals if the toxicity is caused by metabolites that are formed only in humans. Conversely, a drug can be toxic to animals but not in humans if the toxic metabolites are animal specific and are not formed in human.
Hepatocytes isolated from laboratory animals and human therefore represent important experiment systems for the evaluation of possible species differences in drug metabolism and toxicity. The use of hepatocytes is further made practical by successes in cryopreservation. Cryopreserved hepatocytes allow scientists to perform studies with hepatocytes simply by thawing and using the cells, avoiding the time-consuming isolation procedures. Cryopreserved hepatocytes, both from animals and humans, are now available commercially for use in research.
Individual differences between humans in drug metabolism are an established phenomenon. The differences can be caused by genetic and environmental factors. Hepatocytes from one individual can be substantially different from another individual. To aid “normalization” of the individual differences, human-based drug metabolism systems often involve materials combined from different individuals. For instance, one of the major experiment systems for the evaluation of drug metabolism, human liver microsomes, are often prepared from multiple human livers. This approach has been recently extended to human hepatocytes.
A typical procedure to thaw, pool, and re-freeze hepatocytes requires dilution of the cryopreservation medium followed by centrifugation through a high density medium to enrich for viable cells before the second freezing. These can be a laborious procedure.